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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script type="text/javascript">
        var day = parseInt(prompt("请输入间隔的天数", 1000))
        var today = 2;
        var num = day % 7;
        if (today + num > 7) {
            var future = today + num - 7
            alert(day + "天后是星期" + future);
        } else {
            alert(day + "天后是星期" + (today + num));
        }
    </script>
</head>

<body>
</body>

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